The invention relates to a process for preparing aminoplastics and/or phenolics and to the use of the process for preparing melamine resins and/or phenolic resins, in particular melamine-formaldehyde resins, urea-formaldehyde resins, phenolic resins and blends.
Aminoplastics (injection- or compression-moldings cured either directly or during the further shaping of semifinished products) have good mechanical properties, high flame retardancy, heat resistance and solvent resistance, high scratch resistance and good appearance (surface). The starting materials for preparing aminoplastics are low in price, compared with those for polyamides or other engineering plastics.
A conventional process for preparing melamine resin semifinished or finished parts has a series of steps: the starting materials are firstly precondensed batchwise in a heatable mixing vessel to give an amine-formaldehyde resin solution of about 50% strength, the resultant solution is spray-dried to give melamine-formaldehyde resin powder, the resultant resin powder is dry-mixed with fillers in, for example, a plowshare mixer in a further step, then continuously plasticized via heatable calenders to give a compound, and, in further steps, firstly comminuted coarsely, then finely ground; batches are mixed to give uniformity of color and the material is recompacted to give a free-flowing molding composition and finally injection- or compression-molded to give finished parts.
A simplified process for preparing a melamine-formaldehyde resin in a xe2x80x9csingle passxe2x80x9d reactor is described in U.S. Pat. No. 4,458,062. It is significant here that the starting materials are crystalline melamine and solid formaldehyde and that operations are carried out in the absence of water or any other solvent. Solid formaldehyde, however, is more expensive than aqueous formaldehyde solutions. In addition, this process gives melamine-formaldehyde resins which have properties significantly different from those of resins prepared in the presence of solvents.
DE-A 24 51 874 describes an improved process for preparing aminoplastics, in which an aqueous solution of a precondensate, with addition of pigments if desired, is passed into a fluidized-bed dryer and there sprayed onto fillers, dried, and then pelletized and used directly as molding composition.
It is an object of the present invention to provide a continuous process which permits simple and low-cost preparation of aminoplastics in a small number of steps. The novel process should moreover allow the processing and product properties of aminoplastics to be specifically adjusted within a wide range in a simple manner.
We have found that this object is achieved by means of a process in which a precondensate solution is prepared in a first extruder, the precondensate solution is continuously fed, with addition of additives, to a second extruder with partial filling in the feed section (E) and devolatilization section (G), devolatilized in the second extruder and then formed to give an end product.
It has been found that the addition of appropriately chosen additives in the second extruder makes it possible to adjust the reactivity of the resin or to make controlled improvements in product properties, such as in particular impact strength or other mechanical properties. The process moreover allows the residual moisture of the product to be adjusted in the second extruder via devolatilization systems.
In the novel process, a precondensate solution is firstly prepared in a manner known from EP-B 0 355 760, by mixing the starting materials, for example melamine and aqueous formaldehyde solution, and, if desired, paraformaldehyde, in a molar ratio melamine: formaldehyde of from 1:1.5 to 1:3, preferably from 1:2 to 1:3, if desired adding known modifiers and additives which do not disturb the condensation reaction, for example monoalcohols, diols, sulfonamides, sugars, basic catalysts, such as amines, aminoalcohols and hydroxyalkylmelamines, and precondensing them in a first extruder at from 120 to 140xc2x0 C. to a degree of condensation corresponding to a viscosity rise to at least 1 Pa. s. The precondensate solution has a solids content of from 60 to 90%, preferably from 75 to 80%.
Urea-formaldehyde precondensate solutions may be prepared correspondingly from urea solutions and formaldehyde solutions, if desired with suitable additives.
A vital element of the process is a second extruder, to which is fed the precondensate solution prepared in the first extruder in a known manner. While the first extruder must be 100% filled, so that the residence time can be precisely adjusted, giving a defined degree of condensation, the second extruder must, according to the invention, be partly filled in the feed section (E) and in the devolatilization section (G). This makes it possible firstly to add directly into the second extruder additives which can be used to adjust the processing and product properties of the aminoplastics and secondly to carry out, in the second extruder, a devolatilization of the melt, which would not be possible in a completely filled extruder because of the resultant discharge of solids.
The first extruder is preferably operated in a range of rotation rates of from about 12 to 20 rotations per minute, and the second extruder preferably at from 20 to 300 rotations per minute, particularly preferably from 80 to 150 rotations per minute.
The temperature of the second extruder can be controlled section by section via temperature sensors (TIC), the settings being advantageously from 30 to 170xc2x0 C., preferably from 80 to 120xc2x0 C.
At the end of the second extruder, acid catalysts are added which bring about the subsequent curing of the products.
Attached to the second extruder there is equipment which brings about the forming of the end product in a suitable manner:
In a first version, the fully formulated molding composition devolatilized in the second extruder may be extruded via a multi-strand die, pelletized, given a final drying and cooled. The pellets have properties similar to those of commercially available molding compositions and may be further processed, for example by injection molding.
In a second version, a sheet die may be used at the end of the extruder, and through this the raw molding composition may be shaped to give continuous sheet. The process parameters here are adjusted so that the sheet has sufficient dimensional stability but is not yet fully cured. The resultant sheet is then calibrated and cooled. This gives a salable product which is a semifinished product capable of further processing and which may be reshaped in heatable presses to give the desired cured finished part.
In a third version, the continuous sheet is calibrated and completely cured at from 130 to 190xc2x0 C., by attaching a heatable belt press downstream of the sheet die. The fully cured molding can be used in various application sectors, in particular in the electrical, building and motor vehicle sectors.